Field of the Invention
The present invention relates to a three-dimensional measurement apparatus that measures three-dimensional coordinates of a target object, and a control method for the same.
Description of the Related Art
There is a widely-known three-dimensional measurement apparatus that projects a pattern of light onto a measurement target object by using a projection apparatus such as a projector, and obtains the three-dimensional coordinates of the measurement target object by using the principle of triangulation, based on the position of reflection light observed by an image capturing apparatus. Representative examples of three-dimensional measurement techniques available for this kind of three-dimensional measurement apparatus include a light-section method and a spatial encoding method. With such a three-dimensional measurement apparatus, the accuracy in measuring the three-dimensional coordinates greatly depends on the material of the measurement target object as described below.
For example, a measurement target object manufactured from plastic or the like typically causes, for example, a problem in which the measurement accuracy is degraded, or a problem in which measurement cannot be performed in the first place, due to the phenomenon called subsurface scattering or internal scattering of the pattern of light. Therefore, in the case of performing three-dimensional measurement of such a measurement target object, a treatment such as application of a white powder or the like to the surface of the measurement target object is required in advance, and this requirement is a factor that limits the applicable range of the three-dimensional measurement apparatus.
Usually, the following methods are used as methods for correcting the influence of the aforementioned internal scattering. Japanese Patent Laid-Open No. 2012-251893 (hereinafter referred to as Document 1) and Japanese Patent Laid-Open No. 2013-019890 (hereinafter referred to as Document 2) disclose that a table, which shows how the estimated value of depth changes due to the influence of the internal scattering, is preliminarily created using data obtained by measuring an object having scattering properties (internal scattering (subsurface scattering) properties) that are similar to those of the measurement target object. For three-dimensional measurement of the measurement target object, this disclosure makes it possible to correctly measure the three-dimensional shape of the measurement target object including a translucent portion by correcting the value of depth using the table. Also, Japanese Patent Laid-Open No. 2008-281339 (Document 3) discloses a method for performing three-dimensional measurement while eliminating the influence of internal scattering by separating a direct reflection light component, which is reflected off the surface of an object, and an internal scattering component, which scatters inside the translucent object, using a polarizing plate. According to Document 3, it is possible to correctly measure the three-dimensional shape of the measurement target object including a translucent portion by using the direct reflection light component from which the internal scattering component has been removed.
According to both Documents 1 and 2, a measurement value, which is obtained from a captured image obtained by an image capturing apparatus performing image capturing from a single view point, is corrected by using correction information, which has been preliminarily obtained for an object that has the same scattering properties as those of the measurement target object. Therefore, it is necessary to prepare a reference object that has the same scattering properties as those of the measurement target object and to preliminarily perform detailed measurement by other means. In addition, the degree of improvement in the measurement accuracy that can be expected to be achieved by making a correction is limited. Also, there are some surface orientations with which almost no direct reflection light component can be observed, depending on the geometric relationship among the position of the projection apparatus, the position of the image capturing apparatus, the orientations of the surfaces of the measurement target object, etc. Furthermore, there are some cases where almost no direct reflection light component can be observed, depending on the degree of translucence, such as the transmittance, of the measurement target object. Therefore, by the method according to Document 3, it will be impossible to perform three-dimensional measurement if no direct reflection light can be observed as described above. Therefore, by measurement based on the direct reflection light component as disclosed in Document 3, there are at least some measurement target objects that cannot be handled, depending on the material, etc.